Over the last 40 years the need for the therapeutic use of biological materials, such as blood, skin and other tissues, kidneys, hearts, livers and other body organs has increased dramatically. Blood and plasmas components, including red cells, platelets, clotting factors, albumin, and antibodies are isolated and used to treat various bleeding problems. In particular, platelets, essential components of the human blood, are used extensively for assisting in the control of bleeding and replacing functionally defective platelets in patients. For example, platelet transfusions are required by trauma patients who have lost significant amount of blood, patients undergoing chemotherapy that reduces the number of platelets and causes functional defects in remaining platelets, and patients with certain platelet-depleting diseases.
Constituents in whole blood include leukocytes (white blood cells), erythrocytes (red blood cells), thrombocytes, platelets and plasma. Platelets are not entire cells but small detached cell fragments or “minicells” derived from the cortical cytoplasm of large cells called megakaryocytes in the bone marrow. Platelets comprise an outer membrane and cytoplasm from the megakaryocytes which contain granules, dense bodies, dense tubular system and mitochondria. Platelets adhere specifically to the endothelial cell lining of damaged blood vessels, where they trigger and participate in hemostasis, or clotting, and release inflammatory mediators in response to contact with the endothelial cell lining. Important mediators released by platelets include serotonin and coagulation factors. Vascular breaches are repaired by platelets through adhesion and the response to damage is amplified by platelet secretions resulting in platelet aggregation and fibrin formation, i.e. stabilized clot.
It is very important to preserve platelets after their isolation from the body under conditions not only maintaining the biological activity of platelets but also suitable for clinical use. The average survival time for a platelet in the body after it leaves the bone marrow is 8–10 days. The average expected survival time for circulating platelets is 4–5 days, an average of the entire population. The average survival time for platelets after isolation from the body is about 5 days at room temperature.
The current standard and approved method for platelet storage is in a platelet bag at room temperature and limited to five days. The storage time is presumably limited by a decrease in pH due to increased lactate associated with anaerobic metabolic activity. Furthermore, the bag of platelets in plasma must be constantly in motion on a rocker to prevent aggregation. One of the disadvantages associated with preserving platelets under room temperature is the growth of bacteria in the platelet suspension. Platelets in a suspension stored in a refrigerator, albeit with suppressed bacteria growth, tend to activate upon contacting each other and aggregate.
Several approaches such as cryopreservation (freezing) techniques have yielded an increased number of platelets following storage. However, there is a limitation in the functional capacity and persistence of platelets in circulation-that are recovered from such preservation conditions by using these methods. Freezing temperatures require the use of cryoprotectors such as DMSO (dimethyl sulfoxide) (Valeri, Feingold, and Marchionni, Blood, vol. 43, No. 1 (January)1974) and THROMBOSOJ to prevent damage to these biological materials. However, these cryoprotectors are cytotoxic, and typically leave a significant portion of the platelets with either reduced or no functional ability. Moreover, cryoprotectors usually require time-consuming preparation, such as rinsing processes, before the materials can be used, and cryoprotector residues often still remain afterwards. Freezing processes can store erythrocytes for more than 30 days, and leukocytes up to 12 hours only.
Other attempts to preserve platelets have included adding platelets activation inhibitors (Bode, Holme, Heaton and Swanson, Vox Sang, 60: 105–112 (1991); U.S. Pat. No. 5,622,867) or gelatin into the preservation medium (U.S. Pat. No. 2,786,014).
A need continues to exist for a storage system that will store biological materials, particularly platelets, for an extended period of time and still maintains their viability and bioactivity.